Digital VTR's call be expected to receive digital video data in a compressed format. Several formats have been proposed for compressing video data to form a digital video data stream which may then be displayed and/or recorded on video tape. For a discussion of several proposed digital video standards, see U.S. patent application Ser. No. 08/003,887 referred to above.
One digital video compression and data transmission format that offers particular promise with regard to high definition television ("HDTV") is the ISO-MPEG (International Standards Organization--Moving Picture Experts Group) standard described in a report titled "Coding of Moving Pictures and Associated Audio for Digital Storage Media up to about 1.5 Mbits/s", ISO 2 11172 rev 1, June 10, 1992 hereby expressly incorporated by reference.
Terms used in this application are intended to be used in a manner that is consistent with the same terms used in the MPEG standard unless indicated otherwise. Thus, references to video pictures, I-pictures, P-pictures, B-pictures, video codewords, video codeword headers, slices, slice headers, macroblocks, macroblock headers, DCT (discrete cosine transform) coefficients and other terms used to refer to video data stream elements and compression techniques are intended to refer to such elements and techniques as defined by the MPEG standard. The use of such MPEG terminology is, however, in no way intended to limit the present application to the MPEG video data standard. Accordingly, references to MPEG data stream elements are intended to cover similar "MPEG like" data stream elements incorporated into video standards which use the same basic formats and data compression techniques described in the above referenced MPEG documents.
It is to be understood that various features of the present invention, such as data recording techniques, as opposed to, e.g., data prioritization and reduction techniques, are generally not data format dependent and are therefore not limited to applications involving specific data formats.
While digital VTRs may have to be designed to work with one or more video compression schemes and/or data formats, the basic problems associated with increasing the recording time of a digital VTR are generally the same regardless of the format of the compressed digital video data being supplied to a VTR for recording and later playback.
Herein references to normal play modes of VTR operation are intended to refer to modes of digital VTR operation wherein data sufficient to reproduce a complete or almost complete set of the video pictures received by the VTR are recorded on, and/or read from, the video tape. Normal play modes of VTR operation are to be contrasted with trick play modes of VTR operation such as fast forward and reverse operation where only a small portion of the video data received and recorded on a tape are read and displayed during such trick play operation.
Analog video tape recorders that are capable of supporting multiple normal play modes of VTR operation are well known. For example, VHS VCR's generally support long play "LP" mode, and extended play "EP" mode in addition to the standard play "SP" mode of operation. In each of the three modes of operation, the same fixed length of tape is used to store a different number of video images, e.g., a sufficient number of images to display 2, 4, or 6 hours of National Television Systems Committee ("NTSC") analog video data. Each of these three different normal play modes of operation provide differing image quality.
The different normal play modes supported by VHS VCRs are achieved by using different tape speeds for each normal play mode of VHS VCR operation resulting in different data densities on the tape for each normal play mode. This permits the video data rate and the tape output data rate to remain unchanged in the different modes of operation. While the video data rate remains the same for all modes of VHS operation, as the tape data density is increased to support the longer play modes of operation, the signal to noise ratio ("S/N") is decreased resulting in a corresponding decrease in image quality during VHS playback operation.
Digital video tape recorders, including those that might be used to record HDTV will generally be required to operate in the highest tape data density mode possible in order to store the large amounts of digital data needed to represent video images. Thus, any attempt to increase data density on a digital video tape beyond the normal data density will result in an unacceptable digital error rate. Such a high error rate is due to the decrease in the S/N ratio which results from the use of the higher than normal tape data density. In a digital VTR the increased digital error rate that results from the use of higher than normal tape data density rates, is likely to lead to a catastrophic loss of picture. Accordingly, varying the tape data density in a digital VTR does not provide a viable means of supporting multiple normal play recording and video tape recorder playback speeds, i.e., modes of digital VTR operation, as it does in analog VCRs.
The use of data reduction techniques to reduce the amount of data required to represent a series of images might appear to be the only thing necessary for increasing digital VTR recording time. However, the mere reduction, e.g., through the use of data compression or other techniques, in the data rate required to create a series of video images, in and of itself, is insufficient to achieve a long-play mode of operation in a digital VTR. Generally, because of the difficulty of manufacturing a headwheel assembly that can be used to record video data at more than one rotational speed, known digital VTR's only support the recording of a data bit stream at a single constant data density. Because HDTV and other video formats require that a fixed number of video images be displayed during a time period of a predetermined duration, a reduction in the data rate requires that less data be recorded and later read back per a given unit of time than would be required if the data were recorded and read back at the full bit stream data rate. Thus, a digital VTR which implements a long-play mode through a reduction in the data rate is required to implement one bit stream recording and playback data rate for standard play operation and another bit stream recording and playback data rate for long-play mode operation.
Known digital VTRs are capable of recording data comprising bit streams only at a single constant data rate. Accordingly, because known digital VTR's are incapable of recording multiple-speed bit streams at a constant data density, which would be required to support a standard play and a long play mode of operation in a digital VTR, known VTRs can not support a long play mode of operation implemented using data reduction techniques alone.
While some known data logging devices based on linear scan, as opposed to helical scan, recording methods support the recording of multiple-speed bit streams, such data logging devices are impractical for use as digital VTRs. This is because linear scan data recording devices capable of high data rates generally use a large number of heads which make such recording devices too costly for use as consumer digital VTRs.
Accordingly, there is a need for a digital VTR that can support at least one long play mode of operation in addition to standard play operation. In addition, in order to maintain compatibility with standard HDTV receivers during long play mode operation, the digital VTR should generate a data stream that is compliant with the video data compression standards and data stream format used during standard play mode. Furthermore, it is highly desirable that the digital VTR be capable of being implemented in a manner that makes it practical as a consumer digital VTR.